Re: [Haskell-cafe] generating Maybe

On Thu, 8 Nov 2007, Stuart Cook wrote:

On 11/8/07, Tim Newsham wrote:

...

Data.Maybe has functions for processing Maybe's but nothing useful for creating maybe. I think the following would be a very useful addition, a guarded function:

guarded :: (a -> Bool) -> (a -> b) -> a -> Maybe b guarded p f x | p x = Just (f x) | otherwise = Nothing

such a function in the std libs would make functions like "unfoldr" more attractive -- uses of foldr nearly always encapsulate this notion.

How about this variant:

ensure :: (MonadPlus m) => (a -> Bool) -> a -> m a ensure p x | p x = return x | otherwise = mzero

Which as Jonathan points out, could also be written:

ensure p x = guard (p x) >> return x

Now we can define

guarded p f x = ensure p x >>> fmap f

My 'toMaybe' is a specialisation of '\b x -> guard b >> return x'. This let us return to the discussion, whether there should be a specialised function, if there is a general function which does the same. http://www.haskell.org/haskellwiki/Simple_to_complex#Type_class_methods I use 'toMaybe' a lot, because it allows partial application and I have seen library code, where 'toMaybe' could have been used, but its author didn't seem to know or like 'guard b >> return x'.